1. Field of the Invention
The invention concerns the field of reinforced reaction injection molded polyurethanes.
2. Description of the Prior Art
Reaction injection molding (RIM) is a technique for the rapid mixing and molding of large, fast-curing urethane parts. RIM polyurethane parts are used in a variety of exterior body applications on automobiles where their light weight contributes to energy conservation. RIM parts are generally made by rapidly mixing active hydrogen-containing materials with polyisocyanate and placing the mixture into a mold where the reaction proceeds. These active hydrogen-containing materials comprise high molecular weight polyhydric polyethers and low molecular weight active hydrogen-containing compounds. After reaction and demolding, the parts may be subjected to an additional curing step which comprises placing the parts in an ambient temperature of about 250.degree. F. or greater.
For large parts such as hoods or trunk decks of automobiles, RIM polyurethane elastomers may tend to sag and distort, thus placing a limitation upon the uses of these materials in large part applications. One meethod of alleviating this sagging and distortion problem is to place a reinforcing mat in the mold and injecting the polyurethane RIM elastomer resin into the mold so that the mat is impregnated with the RIM elastomer formulation and the resulting part is greatly increased in stiffness so that the sagging does not occur. The technique of placing a mat to reinforce polyurethane materials is not new in itself. Saidla's U.S. Pat. No. 4,073,840 discloses reinforcing mats for use in polyurethane foams. The Saidla materials, however, appear to be rigid polyurethane foams and not elastomeric materials.
Problems heretofore connected in placing reinforcing mats in a mold and subsequently injecting RIM polyurethane resins into the mold resulted in such problems as displacing the mat from its original position and incomplete fill of the mold and/or part distortion caused by the mat not being uniformly distributed throughout the finished composite. These problems are thought to be caused by the liquid RIM reactants attaining an early high viscosity due to fast initial section.
We have discovered a method whereby these problems may be overcome and a complete composite may be obtained which has excellent properties and which does not present processing problems such as incomplete mold fill.